In short-wavelength emitting radiation sources, e.g., EUV radiation sources, the elements involved in the generation of the plasma which are heated to a very high degree by the plasma generation are cooled with a metal cooling liquid in the form of a molten metal.
Cooling circuits with molten metal are already known for a variety of applications such as the cooling of high-output circuits, nuclear reactors or radiation sources for the X-ray range.
Metals coming under consideration for metal cooling liquid are, for example, lithium, gallium, gadolinium, tin or alloys thereof having similar characteristics. Metal cooling liquid has the advantage of very good heat conductivity and, further, has electromagnetic properties so that the pumps that are required for generating a flow of cooling liquid can be realized in a compact manner without moving parts and outside the flow of cooling liquid.
Electromagnetic induction pumps are particularly suitable for this purpose. A closed flow vessel, e.g., a pipe, guiding the metal cooling liquid can be surrounded from the outside with these electromagnetic induction pumps, and the metal cooling liquid can be moved in the flow vessel, for example, through inductive action.
An arrangement for handling a liquid metal in a cooling circuit for cooling a plasma-based radiation source is disclosed in the not-prior-published DE 10 2013 103 668, where a revolving element involved in the plasma generation is cooled by the metal cooling liquid. To this end, the metal cooling liquid is moved in a cooling circuit, wherein a highly heated revolving element is at least partially immersed in an immersion bath with metal cooling liquid. The cooling circuit is completed by a reservoir in which the majority of the metal cooling liquid is received and a heat exchanger by which the metal cooling liquid can be tempered (i.e., cooled or heated). At least between the reservoir and the heat exchanger, there is a pipe connection at which are arranged pumping means which propel the metal cooling liquid through the cooling circuit.
During quasi-continuous (pulsed) operation of the radiation source, the entirety of the metal cooling liquid located in the cooling circuit is constantly kept at temperatures above the melting temperature of the metal used as cooling liquid by the—more or less continuous—injection of heat by the revolving element. In order to maintain the cooling effect in continuous operation, the increasingly heated metal cooling liquid runs through a heat exchanger incorporated in a secondary cooling circuit before being supplied again to the revolving element.
However, when the plasma generation of the radiation source is interrupted for some time there is no injection of heat. During these times, it may be necessary to keep the metal cooling liquid at temperatures above the melting temperature of the metal by means of additional heating. To this end, the reservoir is provided with a heater for heating the metal cooling liquid.
If the plasma-based radiation source is taken out of operation completely, the metal cooling liquid in the cooling circuit solidifies. Before restarting the plasma-based radiation source, the solidified metal cooling liquid must first be brought to the liquid state again. For this purpose in the above-cited DE 10 2013 103 668, in addition to the heater provided in the reservoir with which the solidified metal cooling liquid can be melted again proceeding from the reservoir, the heat exchanger (referred to therein as tempering unit) can also be used so that the metal cooling liquid can be melted by heating at two locations.
However, during the melting in the vessels of the cooling circuit it must be taken into account that the solidified metal cooling liquid expands considerably during heating before liquefying. In order not to damage the cooling circuit in this way, special constructional steps are required and a determined time regime must be maintained during melting, which makes the startup of the metal cooling liquid circulation time-consuming, or heating elements must be arranged at all possible parts of the vessel, including all of the pipe connections.